1. Field of the Invention
The present invention relates to a hydraulic control system for a steplessly variable transmission used on an automotive vehicle, and more particularly, to a hydraulic control system for varying effective diameters of pulleys driven by an endless belt so as to vary the ratio of transmission of a steplessly variable transmission used on an automotive vehicle.
2. Description of related Art
One known type of automatic transmission is a steplessly variable transmission having a belt-pulley transmission mechanism which consists of a pair of, or primary and secondary, pulleys variable in effective diameter or pitch and an endless belt rotationaly coupling the primary and secondary pulleys together. The primary pulley is connected directly, or indirectly, to an engine output and the secondary pulley is connected to a drive line of the automotive vehicle. The primary and secondary pulleys are controlled by means of a hydraulic control system to be complementarily changed in effective diameter, thereby steplessly varying the ratio of transmission of the belt-pulley transmission mechanism. Such a hydraulic control system for steplessly variable transmissions is known from, for example, Japanese Unexamined Patent Publication No. 58(1983)-88252.
The hydraulic control system described in the above publication has as an object to help lengthen the lifetime of the endless belt and improve the engine output transmission efficiency of the steplessly variable transmission. Such is accomplished by regulating the pressure of oil for controlling the effective diameters of the pulleys according to the reduction ratio of the belt-pulley transmission mechanism and an engine torque drawn from engine output curves memorized in a microcomputer, based on an engine operating condition. In a steplessly variable transmission, it is essential, in contrast with conventional automatic transmissions, to regulate accurately the pressure of pulley control oil with a high accuracy in order to prolong the lifetime of the endless pulley and improve highly the engine output transmission efficiency of the transmission. A decreased accuracy in regulating the pressure of pulley controlling oil causes some slippage between the endless belt and secondary pulley. This leads to an engine torque transmission loss in the steplessly variable transmission and a decreased durability of the endless belt.
One of various factors due to which the accuracy of regulating the pressure of pulley controlling oil would deteriorate, as pointed out in, for example, Japanese Unexamined Patent Publication No. 60(1985)-49158, is fluctuations in the pressure of oil in cylinders of the pulleys, caused due to centrifugal forces developed by the rotating pulleys. The pulleys, when rotating, develop such centrifugal forces and thereby cause a decrease in their effective diameters. One approach to compensating the changes of effective diameter of the pulleys, due to centrifugal forces caused in the pulleys, described in the above noted publication is to drop the pressure of pulley controlling oil according to the centrifugal force calculated, based on the speed of rotation of the secondary pulley.
Dropping the pressure of pulley controlling oil according to the centrifugal force, on one hand, causes an increase in oil pump efficiency, which contributes to reducing engine load and takes advantage of fuel economy and, on the other hand, allows the endless belt to produce some slippage, which results in an inaccurate change of the transmission ratio of the steplessly variable transmission. It is apparent that, as a speed at which the automobile vehicle runs becomes higher, the endless belt allows more slippage.